The quality and performance of a digital transmitter is determined by the distance over which the transmitted digital signal can propagate without severe distortions. This is typically characterized as the distance over which a dispersion penalty reaches a level of ˜1 dB. A standard 10 Gb/s optical digital transmitter, such as an externally modulated source, can transmit up to a distance of ˜50 km in standard single mode fiber at 1550 nm before the dispersion penalty reaches the level of ˜1 dB. This distance is typically called the dispersion limit. The dispersion limit is determined by the fundamental assumption that the digital signal is transform-limited, i.e., the signal has no time-varying phase across its bits and the signal has a bit period of 100 ps, or 1/(bit rate) for a 10 Gb/s optical digital transmitter.
High speed optically modulated digital signals are composed of pulses of different width. These pulses distort after propagation in a dispersive fiber because they typically have a wide frequency content. Since these constituting frequencies propagate at different speeds in the fiber, they become of out phase and cause the pulses to distort. Use of signals with narrowed spectrum can therefore help reduce the distortion caused by fiber dispersion since the degree of difference in propagation speed within the signal spectrum (i.e dispersion) is in proportion to the spectral width of the modulated signal.
Another technique to overcome the dispersion limit of fiber transmission is addition of transient chirp to the signal. Transient chirps is an increase or decrease in the frequency of the carrier, which occurs at the rising and falling edges of the digital signal in a time much shorter than the bit period. A red frequency shift on rising edge and a blue frequency shift on falling edge, typically called negative chirp, improves the transmission of NRZ signal over standard single mode fiber (SSMF) with positive dispersion. Such transient chirp can be generated by electro-absorption (EA) external modulator with high negative DC bias or small detuning of bandgap of the absorbing layer from incident wavelength. An externally LiNbO3 modulated signal can be also used to generate transient chirp. The example of this pre-chirping technique is shown in FIG. 1 where electro-absorption modulator (EAM) is used for cording as an example. It is well-known that signal generated by EAM is accompanied by transient chirp as shown in FIG. 1. In the example shown in FIG. 1, the leading edge of the pulse shows the sharp increase in the instantaneous frequency (red chirp) and the trailing edge of the pulse shows the sharp decrease in the instantaneous frequency (blue chirp). In an anomalous dispersion fiber, the blue chirp component propagate faster than red chirp component, therefore, the pulse compresses rather and disperses after fiber transmission (+1600 ps/nm shown on the left).
Transient chirp can also be obtained by forming an external cavity laser consisting of semiconductor optical amplifier (SOA) chip to provide gain and an external fiber Bragg grating (FBG) to form the laser cavity (FIG. 2). The facet reflectivity at the end of SOA facing the FBG is antireflection coated to suppress the lasing within the SOA facets. Lasing in a single mode is possible by using FBG whose reflection bandwidth is narrower than the mode spacing, which is inturn determined by the cavity length. The laser output is modulated with digital data by by directly modulating the injection current into the semiconductor gain section. A variety of gratings can be used instead of the FBG such as a waveguide grating made in Si or Silica, or other planar light wave circuit (PLC) material.